Document classification is an information science problem. In document classification, the object is to assign an electronic document, based on the electronic document's contents, to one or more categories. Document classification has been divided into two types, supervised document classification and unsupervised document classification. In supervised document classification, some external mechanism (e.g., human feedback) provides information on the correct classification for the document. In unsupervised document classification, the classification is done entirely without reference to external information.